The following prior art description outlines the background of the invention mainly in relation to infrared (IR) spectroscopy, it being understood that the present invention is also applicable in spectroscopy employing light in other ranges such as ultraviolet and visible light.
Spectroscopy is widely used nowadays in qualitative and quantitative analysis of materials. Often infrared detection techniques are advantageous over spectroscopic techniques using radiation of shorter wavelengths, such as visible light, since organic and biological materials have characteristic sharp and relatively narrow absorption peaks in the IR region.
In the performance of a direct IR spectroscopic analysis a light beam is passed across a sample material, and the transmission is measured as a function of wavelength which yields a characteristic spectrum. The measurements may be direct and yield an absorption spectrum or indirect and yield an emission spectrum.
The term "measurement" as used herein applies generally and indiscriminantly to detection, identification and quantitative measurement.
Common IR spectroscopic techniques suffer from an intrinsic problem which is due to the strong absorption of IR radiation. Because of this the substrate materials have, as a rule, to be diluted by a medium that is transparent in the infrared, e.g. a liquid such as Nujol (Trade Name) or a solid salt such as KBr. The ensuing high dilution of the sample may give rise to inaccuracies in the results.
An alternative method of direct spectroscopic measurement is the so-called Attenuated Total Reflection (ATR) method. This technique of recording the optical spectrum of a sample material uses an uncladded waveguide for the determination of the concentration of a test species dispersed in a liquid, solid or gaseous medium having a lower refractive index than the waveguide. It is based on total internal light reflection producing an evanescent light wave propagating along the waveguide/test medium interface and it measures the modulation of the evanescent light wave. The ATR technique enables to obtain accurate spectroscopic measurements with smaller amounts of sample materials than in common IR spectroscopy.
Analytical spectroscopic methods employing total internal reflection in waveguides (not ATR) are described, for example, in U.S. Pat. Nos. 4,447,546 and 4,558,014 and in WO 83/01112. These publications describe fluorimetric measurements excited by total internal reflection fluorescence techniques.
The depth of penetration of the evanescent wave into the substrate medium is strongly dependent on the incidence angle of the internally reflected light, and the closer this angle is to the critical angle (beyond which there is no total reflection), the depth of penetration increases exponentially.
The intensity of the interaction between the light travelling inside the uncladded waveguide and the medium surrounding it, is dependent on the concentration of the solution; the intensity of absorption; the depth of penetration (which itself is dependent on the incidence angle of the internally reflected light); and the number of the internal light reflections per unit length which in turn is inversely proportional to the transversal dimension of the waveguide. An increase in the number of the internal reflections amplifies linearly the interaction of the light with the surrounding substrate whereas an increase of the incidence angle amplifies the interaction exponentially.